vapor density

Weight of a unitvolume of gas or vapor compared to (divided by) the weight of an equal volume of air (or, sometimes, hydrogen). Substances lighter than air (such as acetylene, methane, oxygen) are said to have vapor densities less than 1.0 and substances heavier than air (such as butane, chlorine, ethane) are said to have vapor densities higher than 1.0. Whereas all gases and vapors mix with air, the lighter substances tend to rise and dissipate, and the heavier substances tend to concentrate in low places along floors, sewers, trenches and may createfire and health hazards.